Although the cytoplasmic receptor for progesterone has been studied extensively, the nuclear receptor, which is tightly bound to chromatin, has received much less attention because it is more difficult to study. We wish to determine the characteristics and requirements for the binding of progesterone to nuclear preparations of rat, rabbit and guinea pig uteri and some of the properties of the nuclear receptor. Progesterone is reduced to 5 alpha-pregnan-3,20-dione in the nuclear fraction of whole uterus. Beta-Estradiol, which induces the formation of the cytoplasmic progesterone receptor, also enhances the metabolism of progesterone in the nucleus. We will try to determine whether the metabolism of progesterone is involved in the release of the hormone from the nuclear receptor and the replenishment of receptor in the cytoplasm. 1. We will try to correlate the time course of progesterone metabolism with the concentrations of receptor concentrations in the cytosol and nuclear fractions. 2. We will correlate receptor concentrations in the cytosol and nuclear fractions with C14-leucine incorporation into proteins of these fractions.